- Copy stable/10@296371 to releng/10.3 in preparation for 10.3-RC1
builds.
- Update newvers.sh to reflect RC1.
- Update __FreeBSD_version to reflect 10.3.
- Update default pkg(8) configuration to use the quarterly branch.

Approved by: re (implicit)

MFC r286773:
Improve the realtime properties of USB transfers for embedded systems
like RPI-B and RPI-2.

MFC r266396 and r267183:
Build fixes. Add support for fast interrupts.

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Add defines to more easily allow a single threaded version of the FreeBSD
USB stack. This is useful for non-kernel purposes, like the loader.

Initial version of libusbboot, a fully stand-alone, single threaded and
functional compilation of the FreeBSD USB stack for use with boot loaders
and such.

Discussed with: Hiroki Sato, hrs @ EuroBSDCon